Lenses in a single-lens reflex camera may be easily switched because the photographic lens is used as the objective of a finder. Even when photographing an object at the telephoto end or at close range, parallax does not occur. The single-lens reflex camera, therefore, is popular among those cameras whose lenses can be changed.
A finder of a single-lens reflex camera is a real image type of finder through which a magnified real image of an object formed by a camera lens is observed through an eyepiece having a positive refractive power. A generic single-lens reflex finder erects the image of an object with a so-called "quick return mirror," which is a surface mirror, together with a pentagonal roof prism. It is, then, magnified with an achromatic composite positive eyepiece.
Recently, due to the dramatic advancement of plastic foaming technology, plastics have been applied to optical components with even stringent specifications. One of these optical components includes a Daha mirror, a so-called "pentagonal mirror." With this pentagonal mirror, a light-weight and inexpensive single-lens reflex camera can be manufactured.
However, in a pentagonal mirror finder a mirror is used in place of an optical component constructed with a prism. Therefore, it has a substantially long optical path. As a result, the finder has only a low magnification capability.
In order to resolve the problem, Japanese Laid-open Patent Application (Kokai) H4-37404, discloses an eyepiece of the "retro-focus" type, which comprises, in order from the eyepoint end (eye end), a negative lens and a positive lens. The eyepiece of conventional technology has a short focal length for the overall eyepiece while occupying some back focus, the space required to install a pentagonal mirror. However, the eyepiece of conventional technology disclosed in the above Japanese Kokai has about 4.phi. pupil diameter. If the pupil of a photographer, using the camera, is open in a dark place, or the photographer's eye is off center from the exit pupil, it is hard on the photographer's eye to see through the finder.
The apparatus incorporating the principles of the present invention provides a high-performance eyepiece which is capable of providing a high-power magnification and provides a large pupil diameter when it is used as a pentagonal mirror finder.